Pressure bar for veneer cutting

ABSTRACT

A pressure bar of the fixed type for veneer cutting is heated to reduce frictional drag against the wood surface. In the preferred form, the pressure bar is contoured having a tip with a relatively small radius of curvature and a face, in advance of the tip, which has a relatively large radius of curvature. The combination of a heated contoured pressure bar provides performance comparable with that of a roller bar in terms of roughness and friction, but at significantly reduced cost.

BACKGROUND OF THE INVENTION

This invention relates to the cutting of veneer, and particularly to animproved pressure bar for a veneer cutting apparatus.

In the production of veneer, which can be by rotary peeling of a bolt orby slicing from a flitch, it is common practice to use a pressure bar,or nose bar, which is pressed against the wood surface near the point ofcontact of the veneer cutting blade to prevent uncontrolled splittingahead of the cutting edge and to limit the depth of tension checksformed by the wedging action of the blade tip. Pressure bars in use areeither the roller or fixed type.

The fixed pressure bar is considerably simpler, requiring lessmaintenance and is less expensive than a roller pressure bar. However,conventional fixed pressure bars are not entirely satisfactory for thepeeling of softwoods. Fixed pressure bars impose relatively high dragdue to friction between the bar face and wood surface. High frictionaldrag means more torque is required for turning the bolt, making it moredifficult to peel close to wood defects, such as ring shakes and splits,without having the bolt break. Frictional drag also increases thetendency for lathe chucks to spin out. Conventional fixed pressure barsalso tend to dislodge slivers from softwood, and these slivers and otherdebris accumulate along the length of the bar. This accumulation leadsto overcompression and the production of scored and/or rough, furryveneer. To avoid these difficulties, veneer lathes for cutting softwoodsare fitted with the more expensive roller pressure bars.

SUMMARY OF THE INVENTION

It has been found that the frictional drag of a fixed pressure bar canbe significantly reduced by heating the wood contacting surface of thepressure bar.

It was further found that further improvements are obtained if thepressure bar has a contoured tip and face rather than a sharp tip andflat face as in conventional fixed bars. The combination of a pressurebar that is heated and contoured, in addition to reducing frictionaldrag, reduces surface roughness providing the advantages of a roller barbut at considerably reduced cost.

In accordance with one aspect of the present invention, the frictionaldrag of a fixed pressure bar is reduced by heating the wood contactingsurface of the pressure bar. Preferably the wood contacting surface ofthe pressure bar is heated to a temperature of from 70° to 250° C.

In accordance with another aspect of the invention, the pressure bar iscontoured with a tip portion having a small radius portion and a faceportion having a relatively larger radius of curvature. Preferably, thetip portion will have a radius of curvature of from 0.005 to 0.4 inches,and the face portion a radius of from 1 to 6 inches.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a veneer lathe incorporating apressure bar in accordance with the present invention.

FIG. 2 is a cross-sectional view of another embodiment wherein astandard roller pressure bar is modified in accordance with the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a portion of a veneer lathe 1 having a knife 2 and apressure bar 3 in accordance with the present invention mounted on acarriage 4. Veneer 5 is shown being cut from a bolt 6.

The pressure bar 3 is provided with a passageway 7 for conducting aheating fluid such as steam. The heating fluid heats the wood contactingsurface 8 of the bar which reduces the friction of the bar with respectto the surface 9 of bolt 6 as the veneer 5 is being cut.

Although the reason for friction reduction is not understood withcertainty, it is believed that the effect may be due, at least in part,to thermal softening of the wood surface by the heated surface of thebar.

Improvements are achieved as the temperature of the wood contactingsurface is raised above ambient temperatures. The preferred range isfrom 70° to 250° C. The most significant improvements were obtained attemperatures of from 150° to 200° C. As temperatures are increasedfurther frictional drag increases and degradation of the wood results.The reduction of friction obtained by heating means that less torque isrequired to turn the bolt and reduces bolt breakage and the tendency ofthe lathe chucks to spin-out.

The wood contacting surface 8 of the bar 3 is contoured with a tipportion 10 having a small radius of curvature, and a face portion 11, inadvance of the tip 10, having a relatively large radius of curvature.The face portion 11 in advance of the tip 10 allows slivers to be sweptpast the pressure bar which in conventional fixed bars tend toaccumulate at the tip and results in a rough veneer surface.

Preferably, the wood contacting surface of the pressure bar will beprovided with hard wearing material such as chrome plating.

The radius of curvature of the tip 10 should preferably be less than 0.4inches. Optimum results were obtained with a radius of about 0.03inches. To facilitate maintaining a chrome plated surface, the radiusshould be greater than 0.005 inches.

The radius of curvature of the face portion 11 should preferably be inthe range of 1 to 6 inches. A radius greater than 6 inches presents anexcessively high face area in contact with the wood surface and anincrease in frictional drag. Optimum results were obtained with a radiusof 2.5 inches.

The surfaces 12 and 13 of the pressure bar 3 which are in contact withthe carriage 4 are provided with grooves to reduce the contact area andthus heat transfer to the carriage which may be sensitive to thermalexpansion.

The passageway 7 of the pressure bar 3 is provided with grooves 14 toincrease heat transfer to the wood contacting surface 8.

It will be understood that the pressure bar of the present invention maybe heated by means other than a heated fluid such as steam or oil, forexample by electric heating.

FIG. 2 shows an alternate embodiment of the invention, in the form of amodified roller bar 20. The bolt contacting roller of a conventionalroller bar is replaced by a tubular element 21 that is fixed. As in theembodiment of FIG. 1, the wood contacting tip 22 is heated by passingsteam through the passageway 23 of the tubular element 21. The upper lip24 is modified to define the face portion. In combination, the tip 22 oftubular element 21 and the lip 24 provide a contoured surface withcompound curvature similar to the embodiment of FIG. 1. The tubularelement may be rotated periodically to provide a new contacting surfacefor the tip 22.

EXAMPLE 1

Tests were conducted to determine the effects of temperature onwood-to-steel friction. The tests were conducted on western hemlock andspruce, two of the most common softwood species used by industry inWestern Canada. The table below shows the effect of differenttemperatures on the friction coefficient between a tangential surface ofgreen wood and a chrome-plated steel surface.

    ______________________________________                                        Temp. of chrome-plated                                                                            Friction coefficient                                      steel surface °C.                                                                          Hemlock     Spruce                                        ______________________________________                                         20                 0.492       0.512                                          90                 0.228       0.244                                         110                 0.235       0.217                                         150                 0.204       0.210                                         200                 0.220       0.201                                         250                 0.276       0.259                                         300                 0.294       0.278                                         ______________________________________                                    

The results indicate that as the temperature of the steel surface rosefrom 20° C. to 90° C., friction decreased by more than 50% for both woodspecies. Maximum friction reduction occurred between 150° and 200° C.

EXAMPLE 2

An experimental pressure bar was constructed for a 66 inch laboratoryveneer lathe. The pressure bar was provided with a passageway for steamsimilar to that shown in FIG. 1.

Tests were conducted to compare veneer quality, particularly surfaceroughness, using four different types of pressure bar types, namely: aconventional type roller pressure bar, a contoured fixed bar withoutheating; a contoured fixed bar with heating with steam at about 150° C.;and a contoured fixed bar with heating at about 150° C. and havingorifices that allowed steam to impinge on the bolt surface. The lattertwo were provided by using interchangeable caps for the pressure bar,one without orifices.

Ten western white spruce bolts were peeled with each type of bar. Veneerthickness was set at 1/10-inch and cutting speed maintained at 150 feetper minute. Lathe settings were held constant, with the exception thatthe roller pressure bar required a larger vertical gap. Operatingtemperature for the heated pressure bar was 150° C.

Veneer samples were selected from the sapwood, heartwood and corewoodareas of each bolt and measured for surface roughness, thickness, andlathe-check depth.

Veneer surface roughness was found to be the critical factor forselecting the pressure bar which yielded the best veneer quality.Thickness and lathe-check depth were not greatly affected by bar design.

A roughness depth of 0.020-inch was considered as the maximum depth foracceptable veneer. In the test, all veneers with roughness deeper thanthis limit were considered degrade and recorded as a percentage of thetotal veneer produced.

Of the four pressure bar types, the roller bar produced the poorestquality veneer. Average roughness depth was 17.5 thousandth inch and20.7% of this veneer was beyond the 0.020-inch roughness tolerancelimit.

The contoured pressure bar without heat produced significantly betterveneer quality, with an average roughness depth of 14.3 thousandths inchand only 4.1% of the veneer beyond the limit.

The best veneer quality for the study was produced by the two heatedpressure bars. The contoured steam-heated bar produced a roughness depthof 12.5 thousandths inch, with only 1.4% of the veneer outside thetolerance. Veneer from the contoured steam-injection bar was slightlyrougher, with 12.7 thousandths inch depth and 3% off tolerance.

All three of the contoured pressure bars gave better results than thestandard roller bar. No problems with sliver buildup or veneer scoringwere observed with any of the contoured pressure bars.

EXAMPLE 3

A pressure bar similar to that shown in FIG. 1 was installed in anindustrial Premier lathe. The pressure bar had a tip radius of 1/32inches and a face radius of 2.5 inches. The tip defining member and facewere plated with chrome. Steam was applied at various temperaturesbetween 115° and 177° C. Testing involved a variety of softwood speciesfound in Western Canada. The best results were obtained between 155° to165° C. Operation at these temperatures produced results similar to thatof a conventional roller bar, that is, there appeared to be no qualitydifference in roughness or lathe check depth, and no increase inspinouts or decrease in yield.

We claim:
 1. In a system for cutting veneer using a fixed pressure bar,the improvement comprising heating the wood contacting surface of thepressure bar to a temperature of from 70° to 250° C. to reduce thefrictional drag on the wood.